The length of the screw an the extruder is typically rather great with respect to the diameter of the screw. It is very difficult to mount in bearings a long screw. No matter how firmly the screw is mounted in bearings from one end, in difficult situations the screw can still bend to such an extent that it touches a cylinder positioned outside, which immediately leads to fast wearing. Arrangements have been disclosed where a long screw is also mounted in bearings from the end near the nozzle in such a way that melt plastic flows through the bearing. Such a bearing arrangement does prevent the screw from bending, but it leads to disadvantageous weld lines in the plastic mass, thus producing a weak spot in the product to be extruded.
When products having a very slippery surface are to be prepared, a large amount of slip agent must be mixed with the plastic mass to be extruded. However, a large amount of slip agent, such as silicone oil, that does not dissolve in plastic mass also results in the friction between the screw and the plastic mass being small, whereupon only a little heat is generated by the friction. The small friction results in the mass sliding in some places in the groove of the screw against the cylinder with perfect glide. However, since the plastic mass sticks in other places, a very strong bending moment easily occurs in the screw. Therefore the use of the very slippery plastic mass described above surprisingly leads into a situation where the screw wears greatly and the screw must be firmly mounted in bearings.
DE 1 961 078 discloses an extruder comprising a conical stator and a rotatable conical rotor placed outside the stator. The material to be extruded is fed between the stator and the rotor. The outer edge of the rotatable rotor is mounted to the frame with roller bearings in such a way that the bearings receive axial forces that are directed downwards in the figure. The rotor is also mounted with rolling bearings, which receive radial forces. This extruder cannot be used in a situation where the rotor is subjected axially to a force directed upwards in the figure. Further, it is not possible to monitor the forces acting on the rotor, nor to control and regulate them.
EP 89 906 779 discloses an extruder comprising at least three conical stators and at least two conical rotors placed between the stators. The material to be extruded is fed between the rotor and the stator via supply conduits. The lower surface of the rotors is mounted in bearings against a lower stator. This bearing arrangement receives the forces directed downwards, i.e. when the pressure outside the rotor is greater than the pressure inside. This bearing arrangement cannot receive radial forces or forces that are directed upwards. It is not possible either to monitor and regulate in any way the forces acting on the rotor.
The purpose of the present invention is to provide an extrusion method and an extruder comprising none of the aforementioned drawbacks.